The present invention is intended for use in a scanning output system that uses a beam for generating information, or more specifically, wherever a digital pulse forming circuit may be used to control a scanning beam. The beam may vary in intensity and duration according to the pulses used to control the beam. For example, a laser beam may be used in a printer for selectively exposing areas on a photoreceptor. The latent electrostatic image formed on the photoreceptor by the beam exposure attracts developing toner in proportion to the latent image charge level to develop the image. As another example, a cathode ray tube uses an electron beam to scan a phosphorous screen. The electron beam may be varied in intensity and duration to accurately display information on the phosphorous screen. In both examples, a pulse forming circuit may be used to generate video pulses to control the intensity and operation time of the respective beams.
Heretofore, various methods and apparatus have been used to vary the position and width of pulses used to control laser or CRT beams. The following disclosures may be relevant:
U.S. Pat. No. 4,347,523, also to Ohara, discloses an apparatus of general interest which uses an input signal to address pulse numbers with corresponding pulse width selection numbers.
U.S. Pat. No. 4,375,065 to Ohara describes an apparatus of general interest that uses pulse number and pulse position modulation to control a laser beam.
U.S. Pat. No. 4,390,882 to Ohara et al. discloses for an image processing apparatus a method of adjusting the image density by controlling the on time of the laser. Control of the laser on time is performed by a multivibrator having a variable RC time constant.
U.S. Pat. No. 4,544,264 and U.S. Pat. No. 4,625,222, both issued to Bassetti et al. describe enhancement circuits suitable for use in a laser based electrophotographic printing machine. The enhancements are directed at modifying the digital drive signals used to produce the image, including smoothing digitized edges and broadening fine lines in both the horizontal and vertical directions. Leading and trailing edge signals, in both directions are provided to potentially print each black pixel or line as a series of three pixels, a gray leading pixel, overlapped by a central black pixel, which is in turn overlapped by a gray trailing pixel. A similar process is applied for scanlines as well. The series of signals are recombined to effectively control the voltage and current levels of a laser driver.
U.S. Pat. No. 4,544,922 to Watanabe et al. teaches a smoothing circuit for an orthogonal matrix display. The circuit adds or removes a "small dot" on the display from either the first or last third of a dot clock (DCK) period which is one-third the period in which a standard dot of the original pattern is displayed.
U.S. Pat. No. 4,626,923 to Yoshida teaches an image processing apparatus for producing a halftone image in which the on time of the laser is controlled by both the image input data and a pulse width modulation circuit. The image data is transferred under control of clock signal, CLK. The pulse width modulation circuit includes a clock, CLKH, having a frequency three times that of CLK, which is used together with latches and AND gates to provide synchronous sub-pixel addressing.
U.S. Pat. No. 4,661,859 to Mailloux et al. describes an image processing circuit for producing a greyscale image in which the on time of the laser is controlled by both the video input data and the pulse width modulation circuit. The pulse width modulation circuit includes a clock having a frequency greater than the video data rate, which allows synchronous sub-pixel addressing.
U.S. Pat. No. 4,905,023 to Suzuki, describes an image forming apparatus using a plurality of conversion tables addressed by an input video image signal to generate pulses.
U.S. Pat. No. 4,926,268 to Kawamura et al. discloses an image processing apparatus which employs analog circuitry to produce a pulse-width modulated (PWM) output from a multi-level digital signal. As described, each analog signal is generated in synchronism with the pixel clocks.
U.S. Pat. No. 4,933,689 to Yoknis describes a method for enhancing a displayed image in a laser exposed dot matrix format to produce softened edge contours. Using three pulses, a central pulse plus leading and trailing enhancement pulses which are separated therefrom. The purpose of the leading and trailing pulses is to create a blurred or grayed region at the leading and trailing edges of each associated character.
U.S. Pat. No. 4,965,672 to Duke et al. discloses an apparatus for varying the width and position of pulses used to control a laser beam.
U.S. Pat. No. 5,144,337 to Imamura et al. teaches an image forming apparatus suitable for changing the size of an output dot in a main and subscanning direction. Dot size and shape are controlled by pulse width modulation and power modulation applied to a laser diode.
U.S. Pat. No. 5,144,338 to Sakano discloses an image recorder which employs a pulse width modulated laser beam to control the recording position on a photoconductive drum. The position (left aligned, centered, or right aligned) and duration (12 ns, 20 ns, 32 ns, or 56 ns) of the pulse within a pixel interval is determined based upon the tone level of the pixel of interest and its relation to the tone levels of both preceding and following pixels.
U.S. Pat. No. 5,184,226 to Cianciosi describes a digital system for generating pulses from a series of data words, the relevant portions of which are hereby incorporated by reference. The system employs multiple RAM look-up tables for translating the data words into a series of corresponding pulses utilizing two channels to achieve the desired throughput.
U.S. Pat. No. 5,193,011 to Dir et al. discloses a system for printing gray levels without the need of a halftone cell. The system determines the pulse width for each pixel as a function of the gray level of the pixel, based upon an iterative comparison to an incrementing grey level clock. In one embodiment, a page-wide liquid crystal shutter is used to regulate the exposure of a photoconductive drum. The shutter may be toggled on and off multiple times for each pixel during the recording of a single row of the image.
L. Steidel in "Technology Overview: Resolution Enhancement Technologies for Laser Printers", LaserMaster Corp., discusses three currently available implementations for vertical resolution enhancement, Resolution Enhancement Technology, Paired Scan Line Scheme, and TurboRes. In all cases, the horizontal resolution of the laser scanner is increased by increasing the clock speed. On the other hand, the vertical resolution is enhanced by combining the weaker laser laser energy from a brief laser flash, which leaves only residual or fringe energy on the image drum at the periphery of a pixel of an adjacent pixel on a second scan line.
An object of the present invention is to provide a pulse width position modulation system having the capability to selectively produce multiple pulses within a predefined pixel clock period without the necessity of increasing the speed of the hardware used to produce the pulses.
Another object of the present invention is to enable the pulse width position modulation system to selectively produce pulses which are justified with the beginning and end of a predefined pixel clock period so as to enable the extension of adjacent pulses produced in preceding or succeeding clock periods.
In accordance with the present invention, there is provided an apparatus for generating multiple pulses within a predefined clock period. The apparatus comprises means for specifying the leading edge delay for a first pulse to be generated during the predefined clock period, means for specifying the trailing edge delay for the first pulse to be generated during the predefined clock period, means for generating the first pulse during the portion of the clock period between the leading edge delay and the trailing edge delay, and means, selectable on a clock period basis, for inverting the first pulse to produce a pair of complimentary pulses within the selected clock period.
In accordance with another aspect of the present invention, there is provided a digital electronics system capable of generating pulses within predefined clock periods which delimit pixel boundaries. The system includes an apparatus for extending the pulses beyond the pixel boundaries, comprising, means for specifying the leading edge delay for a first pulse to be generated during the predetermined clock period, means for specifying the trailing edge delay for the first pulse to be generated during the predetermined clock period, means for generating the first pulse during the portion of the clock period between the leading edge delay and the trailing edge delay, and means, selectable on a clock period basis, for inverting the first pulse to produce a pair of complimentary, boundary justified pulses within the selected clock period, said boundary justified pulses thereby extending the pulses generated during a preceding and a succeeding clock period.
In accordance with yet another aspect of the present invention, there is provided a digital electronics system for generating pulses from a series of data words, comprising means for translating the series of data words into a series of pulse attribute words, wherein each pulse attribute word includes information for controlling the formation of a corresponding pulse during a clock period, means for splitting the series of pulse attribute words into two channels, means corresponding to each channel for accepting pulse attribute words from respective channels, and forming pulses using the information included in the pulse attribute words for controlling the formation of the pulses, means for generating the pulses, wherein a pulse from a first pulse forming means is generated while a pulse from a second pulse forming means is being formed, and means for inverting the pulse generated during a selected clock period to produce a pair of complimentary pulses within the selected period.